The present invention relates to a method for forming a phosphor screen of a cathode ray tube, more particularly to an improvement of a phosphor powder coating method.
In a color cathode ray tube, three different phosphors that respectively emit blue, green and red light are coated in a regular array (in a set pattern) in the form of stripes or dots to the inner surface of a panel which displays an image. Conventionally known methods for forming such phosphor coating layers include a phosphor slurry method and a phosphor powder coating method. The phosphor slurry method uses phosphor in a slurry mixed with photoresist material. In forming a phosphor screen, such a method inconveniently uses lot of water for the development.
The phosphor powder coating method recently developed however has a benefit free from such water development. In the powder coating method, as disclosed in Japanese Patent Publication No. 57-20651, for example, a thin layer of an aromatic diazonium salt, or a photo-tacky composition which contains an aromatic diazonium as a photosensitive component and occurs adhesivity on exposure to light is formed on the inner surface of a panel. Phosphor particles are brought into contact with and held in the thin layer of which a sticky surface formed in the exposed portion, and then remaining excess phosphor particles are removed from the thin layer, whereby a patterned phosphor layer is formed.
In this method, first, a photo-sticky or photo-tacky material, i.e. a photosensitive composition containing a diazonium salt, is coated on the inner surface of the panel in the form of an aqueous solution, heated and dried to give a solid thin layer. The usual procedure in this case is that the panel is coated with an aqueous solution of the photosensitive composition while rotating at low speed and subsequently the speed of rotation is increased, so as to throw the photosensitive composition aqueous solution off the panel. Then the coated solution is heated and dried to form a solid thin layer on the panel are effected by an infrared heater, facing the panel and raised the panel temperature to about 50.degree. C. Next, ultraviolet ray irradiation (exposure) is effected through a shadow mask assembled with the panel. In this case, the panel is cooled beforehand to 30.degree.-40.degree. C., this being done to prevent heat of the panel causing heating and thermal expansion of the shadow mask and consequent shift of positions at which exposure is to be effected. The ultraviolet ray irradiation positions in this process correspond to locations that will be impinged by electron beams in order to cause the phosphor to emit light, i.e., they correspond to locations where the phosphor is to be coated. In a portion of the thin layer where is irradiated by ultraviolet rays, there is produced particle acceptable adhesive surface by diazonium salt photolysis reaction. Next, after the shadow mask is removed, a phosphor powder of a first color is brought into contact with the layer, thereby, causing adhesion of phosphor to an amount corresponding to the positions of the particle acceptable adhesive surface thus obtained. In order to contact such phosphor powder on the adhesive surface a phosphor powder sliding method may be employed. Next, excess phosphor powder is removed from the thin layer by air blow or similar means. In this manner, a first color phosphor layer is formed only on the portions that were exposed. Next, positions where a phosphor powder of a second color is to be fixed are exposed via a shadow mask and the second color phosphor powder is adhered only to the exposed portions in the same manner as forming the first color phosphor. Then a phosphor powder of a third color is adhered to the inner surface of the panel by a similar procedure. The above operation results in a panel on which phosphors of three colors triads constituting a phosphor screen are respectively formed at locations which will be struck by electron beams for causing emission of light. In this method, however, there is the problem that the amount of ultraviolet irradiation energy needed for producing a powder acceptable adhesion region in order to effect adhesion of the second and third colors is 1.5-2 times greater than the corresponding ultraviolet irradiation needed for the first color. In other words, sensitivity is lower with the second color and third color on. Even then the second color and third color adhesion patterns are inferior to the first color pattern and the quality of the second color and third color adhesion patterns is lower. For this reason, in the case that the method applies to coat the phosphor in a minute and precise dot- or stripe-shape on the entire surface of panel of the color cathode ray tube,a part of the phosphor dots peels off or fail to thoroughly thick adhere, as a result, causing to deteriorate the quality of the phosphor screen.